1. Field of the Invention
The present invention relates to telecentric projection lenses which are intended to use at a magnification of about 1/10 and which are precise in terms of their F number and have their aberrations well corrected.
2. Description of the Prior Art
In recent years there have been developed and used original scanning methods in which solid image pick-up elements are employed as scanning means for reading apparatus with the solid image pick-up elements are arranged in the image plane as scanning photo-receptor elements to scan the original.
To carry out the above known scanning methods, a projection lens must be used to transmit the original image to the solid image pick-up elements and further a color separation prism must be interposed between the projection lens and the focal plane containing the elements to transmit the color signals of the original to the solid image pick-up elements. If a common non-telecentric lens is used as the projection lens in this case, the off-axial rays are obliquely incident upon the color separation prism and shading is caused thereby. To overcome the problem of shading there is used a projection lens having a telecentric property. By using such lens, the principal rays of the incident light pass through the focal point on the side of object field and therefore the principal rays of the exit light including off-axial rays on the side of image field can run in parallel with the optical axis. This is one effective method already known for overcoming the problem of shading caused by the color separation prism.
However, projection lenses useful for carrying out the above scanning method have to satisfy many requirements at the same. In general, such a projection lens has to satisfy the following requirements:
(1) The F number of the lens should be relatively precise. To carry out a high speed scanning employing solid image pick-up elements it is preferable to increase, as much as possible, the quantity of exposure light to the elements per unit time. On the other hand, it is also preferable to use, as an original illumination lamp, a light source whose illumination is as low as possible. For this reason, the projection lens is required to have a relatively precise F number.
(2) For the purpose of reduction in size of the apparatus, namely reduction of the distance between the original surface and the focal plane, the lens should have a wide angle of field.
(3) The lens should have a high resolving power because the size of solid image pick-up element is very small which is in the order of 15.mu..
(4) The lens should be suitable for use with its apperture efficiency for off-axis being 100%. This is because the distribution of light intensity must be uniform over all of the solid image pick-up elements.
(5) The lens should be able to project the original surface uniformly. In other words, the distortion of the lens should be of low level.
(6) Since a color separation prism must be interposed between the lens and the solid image pick-up elements, the lens should have a long back focal length.